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About flash floods

1/4 Downstream of Melrose-Yamba intersectionNDC 1637/3 - 5 FEBRUARY 1971: Downstream of Melrose-Yamba Drive intersection causeway from east bank (afternoon) by Australian Capital Territory, available under a Creative Commons Attribution Licence 2.0 at http://flic.kr/p/eZP59V

Flash floods occur when soil absorption, runoff or drainage cannot adequately disperse intense rainfall. The most frequent cause of flash flooding is slow-moving thunderstorms. These systems can deposit extraordinary amounts of water over a small area in a very short time. The strong updrafts of air within thunderstorms can suspend huge amounts of rain before releasing a deluge onto the ground. Such rain can reach intensities of more than 100 mm per hour, provided the environment is humid enough to feed sufficient moisture to the storm. Often topography acts to focus thunderstorm development over a particular location, further accentuating rainfall accumulation.

1-in-100,000 year event hits the ACT

Flash floods can and do hit anywhere, anytime. Older ACT residents will recall the tragic events that unfolded in 1971 in Woden Valley.

According to the Australian Emergency Management Knowledge Hub:

On 26 January 1971 widespread flooding occurred in Woden Valley, south-east of Canberra, as a result of a super cell thunderstorm. Rainfall started at approximately 7.30 - 7.50 pm and up to 100 mm was recorded in one hour by private rain gauges in the suburb of Farrer and Torrens.

The rain waters filled the creek that ran beside the Woden Valley freeway, and as debris blocked the valley, the backup of water sent a flash flood onto the freeway. The 200 m wide, 1.5 m deep torrent engulfed more than a dozen cars, and caused the deaths of seven people aged between six and 20. A subsequent Coroner's Report found the deaths to be accidental drowning, and the Department of Interior found that approximately $120,000 of public/government owned property was damaged.

The Bureau of Meteorology estimated the total damage bill at $9 million.

The Canberra Times has covered the official launch of ACT First, reporting that, "Too many Canberra residents are failing to take simple steps to safeguard their families and properties against storms and floods."

The Canberra region has experienced an average of one earthquake per year for the past 50 years; and for most residents this information would come as a surprise. Frequency of occurrence of earthquakes within 20 km of Canberra GPO Seventy-two earthquakes occurred within 20 km of the GPO in the 36 years prior to the end of 1995, and during this period yearly numbers varied between zero and nine with a mean number of 2.0.

Flash floods can occur almost anywhere there is a relatively short intense burst of rainfall such as during a thunderstorm. As a result of these events the drainage system has insufficient capacity or time to cope with the downpour. Although flash floods are generally localised, they pose a significant threat because of their unpredictability and normally short duration.

105 people have been killed by landslide events since 1842 in 100 recorded landslide events (National Landslide Database, 2007). Many of these events resulted from natural phenomenon, though half of those causing death and injury can be attributed to human activity.

Green Cross Australia brought a whole new twist to disaster preparedness with a zombie flashmob in Canberra. Zombie-clad locals invaded Canberra’s Old Bus Depot Markets, performing to Michael Jackson’s Thriller – all to support ACT First. See photos of the event on the Canberra Times...

Severe heatwaves were once expected to hit Canberra about once every 25 years, experts say, but the sweltering conditions will be the third such hot spell in the capital in four years.
The Canberra Times reports the blistering heat is a sign of things to come, with the CSIRO predicting ACT residents could suffer through 26 days a year with a top temperature of 35 degrees or more before the end of the century.

A community of ACT social media users will add to front-line preparations this year to prepare Canberrans for their next natural disaster. Read John Thistelton's report from the Canberra Times on the launch of ACT First.

Storms can happen anywhere, but they are more common in the ACT from September to end of February. Severe storms are more common than any other natural hazard and are responsible for more damages measured by insurance costs than bushfires, floods or cyclones.

During a ‘heatwave’ temperatures are above average for several sequential days and it remains hot overnight. Heatwaves have caused more deaths than bushfires in Australia, and cause losses to crops, livestock and infrastructure.

Many people believe that tornados do not occur in Australia; this is not true, they do and have caused numerous deaths. Most thunderstorms do not reach the level of intensity needed to produce these dangerous phenomena, but they all produce lightning which can cause death, injury and damage. Tornados can and do occur in the ACT.